In recent years, high integration of semiconductor integrated circuits has been developed, and large-scale integrated circuits (LSI) or very large scale integrated circuits (VLSI) have been put into practical use. With such use, the minimum pattern of the integrated circuit tends to be in the sub-micron region and the lithographic technique tends to become finer. For forming a fine pattern, it is essential to use lithographic technique comprising coating a substrate having a thin film formed thereon with a resist, placing as a dust-proofing film a pellicle over the resist-coated substrate to prevent adhesion of a foreign substance such as dust, conducting light exposure to form a latent image of desired pattern, developing the latent image to form a resist pattern, conducting dry etching using the resist pattern as a mask and then removing the resist to obtain a desired pattern.
In the lithographic technique, ultraviolet rays of g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm) are used as exposure light, and with fining of patterns, far-ultraviolet rays, vacuum ultraviolet rays, electron beam (EB), X-rays, etc., which have shorter wavelengths, have been used as exposure lights. Especially recently, laser beams (KrF excimer laser beam of a wavelength of 248 nm, ArF excimer laser beam of a wavelength of 193 nm, F2 laser beam of a wavelength of 157 nm) are paid attention as exposure lights, and are expected to be useful for the formation of fine patterns.
There is no polymer that transmits a light of such ultraviolet region of shorter wavelength, particularly the vacuum ultraviolet (VUV) region such as a region of F2 laser beam of 157 nm, and it is difficult to select a material.
By the way, Bloomstein, et al. have reported that MgF2 and CaF2 are promising optical materials as inorganic optical materials in the field using lights of vacuum ultraviolet region (J. Vac. Sci. Technol. B15, 2112, 1997). In this report, it is also reported that, as an organic polymer material, Teflon (registered trademark) is better than PMMA, PVC and PAA in the transmittance and it exhibits a transmittance of about 83% in case of a film of 0.1 μm thickness. It has been further reported that with regard to siloxane polymers, methylsiloxane exhibits a transmittance of about 70% in case of a film of 0.1 μm thickness, though the transmittance is not so high.
Examples of the resist materials include a copolymer of a bicyclo structure comprising norbornene having bistrifluoromethyl carbinol as a functional group and sulfone (ACS Symp. Ser. 706, 208, 1998), a copolymer of a fluorine-containing ethylenically unsaturated compound monomer such as tetrafluoroethylene and a polycyclic ethylenically unsaturated compound monomer such as norbornene (WO 00/17712), a fluorinated polymer obtained from a compound having an ethylenically double bond in which a functional group of bistrifluoroalkyl carbinol —C(Rf)(Rf′)OH (Rf and Rf′ are the same or different fluoroalkyl groups) is introduced, and a copolymer of this fluorinated polymer and TFE (WO 00/67072).
Although the OH group tends to improve adhesion to the silicon substrate, the OH group present near the fluorine atom is increased in the acidity and thereby reacts with a carboxylic acid residue formed by the acid decomposition to cause gelation.
The dust-proofing film such as a pellicle used for the lithography comprises a pellicle frame made of aluminum or the like and a transparent membrane made of a resin such as nitrocellulose spread on one side surface of the frame, and can be fitted on a mask by, for example, applying an adhesive on the other side surface of the frame. According to the pellicle, introduction of a foreign substance onto the circuit pattern surface from the outside can be prevented, and even if a foreign substance adheres to the pellicle membrane, an image of the foreign substance is out of the focal point in the exposure of the light and is not transferred, so that a trouble hardly takes place.
As described above, use of lights having shorter wavelengths has been promoted with the fining down of the minimum pattern of the integrated circuit, and with this promotion, development of materials of thin films withstanding energy of the exposure light of shorter wavelength has been made. For example, when KrF excimer laser is used as the exposure light source, a fluorine-containing polymer having a relatively small absorption in the far-ultraviolet region, such as a commercially available fluorine-containing resin CYTOP (trade name, available from Asahi Glass Co., Ltd.) or a commercially available fluorine-containing resin Teflon (registered trademark) AF (trade name, available from U.S. DuPont Co.), is used for a pellicle membrane (Japanese Patent Laid-Open Publication No. 39963/1991, Japanese Patent Laid-Open Publication No. 104155/1992, etc.).
In order to solve a problem of light transmission caused by light scattering properties due to the crystallizability of fluorine-containing polymers synthesized from fluorine-containing ethylenically unsaturated compounds such as tetrafluoroethylene and vinylidene fluoride, perfluoro aliphatic cyclic polymers obtained by radical cyclization polymerization of perfluoro bifunctional unsaturated compound monomers or radical polymerization of perfluorocyclic monomers have been proposed (Japanese Patent Laid-Open Publications No. 238111/1988, No. 131214/1989, No. 131215/1989 and No. 67262/1991), and these polymers exhibit sufficient transmission to ultraviolet rays of 193 nm. Further, it has been reported that a thin film of a copolymer of ethylene tetrafluoride and propylene hexafluoride or a polymer having siloxane bond (WO 98/36324) is used as a pellicle membrane when ultraviolet rays having wavelength of 140 to 200 nm are used as exposure light.
The transmittance of the perfluoro aliphatic cyclic polymers to the vacuum ultraviolet rays of 157 nm is relatively good, but because of synthesis by cyclization polymerization, the amount of fluorine contained in the monomer must be increased, and this causes development of water repellency. As a result, adhesion to the substrate is deteriorated. With regard to polymers which are obtained by ring-opening metathesis polymerization and has double bond and in which a part of fluorine atoms in the perfluoro aliphatic cyclic polymers are replaced with hydrogen atoms or organic groups, the light transmittance to the vacuum ultraviolet rays of 157 nm is lowered by light absorption of the double bond, and the absorption coefficient becomes extremely high. Further, because of the presence of the light absorption band at 157 nm, weathering resistance such as resistance to photodecomposition (photodeterioration) under the circumstances becomes worse, so that such polymers cannot be used. Therefore, further development of polymers excellent in both the weathering resistance and the adhesion and having extremely low light absorption coefficient in the vacuum ultraviolet region of 157 nm has been desired.
In order to solve the above problems, the present inventors have earnestly studied fluoropolymers, which are excellent in light transmission, optical properties, electrical properties, heat resistance, adhesion to substrate and light resistance and can be used as base polymers of optical coating materials or resist materials used for optical materials, thin films, lenses and the like. As a result, the present inventors have found that a novel fluorine-containing polymer satisfies various properties required for the optical coating materials or resist materials used for optical materials, thin films, lenses and the like. Based on the finding, the present invention has been accomplished.
That is to say, it is an object of the invention to provide a novel fluorine-containing polymer which can be used for optical coating materials or resist materials used for optical materials, thin films, lenses and the like and satisfies light transmission in the vacuum ultraviolet region of not more than 193 nm, particularly light transmission in the vacuum ultraviolet region of 157 nm, optical properties, electrical properties, heat resistance, adhesion to substrate and light resistance. It is another object of the invention to provide a monomer favorably used for preparing the polymer. It is a further object of the invention to provide a process for preparing the polymer. It is a still further object of the invention to pvovide uses of the polymer.